What Causes A Wintry Mix Of Precipitation?

Ever wonder why sometimes there is freezing rain, freezing drizzle, or sleet in the winter? Why not just snow? What causes a wintry mix of preciptiation?

To better understand why this occurs, the interaction of warm air on precipitation and the depth of the cloud layer (i.e., the amount of moisture/saturation in the atmosphere) need to be examined.

When it comes to temperature, there are three basic considerations that will impact what type of precipitation will fall:

Surface temperature: When at or below freezing (32F or 0C), snow, freezing rain, freezing drizzle and sleet are all possible. When the temperature is above freezing, rain, drizzle, or even some snow (if not too warm) are possible.

Just above the surface: If there is an above freezing layer of air (called a "warm layer") above the surface, any ice falling through this layer (i.e., snow) could melt into a liquid drop, or partially melt in an ice-liquid mix. If the surface temperature is at or below freezing, this precipitation could then reach the ground as freezing rain, or refreeze just above the surface and land as sleet (a "ball" of ice, also called an ice pellet).

Coldest cloud temperature: This coldest temperature within the cloud layer is important as well since it dictates whether there will be any ice created in the cloud when the precipitation process begins. Generally, temperatures need to be at least -10C for ice growth to begin in the cloud. If temperatures within the cloud are between 0C and -9C, the cloud will likely not have ice but rather any precipitation created within the cloud will be in the form of liquid (super cooled droplets or liquid drops with a temperature below freezing).

The depth of the cloud is a contributing factor to whether there will be a chance for accumulating precipitation (such as rain or snow) or lighter precipitation like drizzle. Generally, the deeper the cloud layer is, the better chance that a precipitation process can be initiated (in meteorology these processes are called the collision-coalescence process and the ice-crystal process [Bergeron-Findeisen process]). If the cloud layer is more shallow, typically it supports light precipitation in the form of drizzle or perhaps flurries - if any precipitation occurs at all.

Below is a north to south (from left to right) running x-section of temperature from a mixed precipitation event that occurred on December 1, 2007. This example illustrates how the temperature a loft (not just at the surface) effects what type of precipitation will fall. Note the strong warm layer intruding from the south and how precipitation changed from rain, to freezing rain, to sleet and then snow depending on how far north the warm layer extended.